I read your question a second time & may have misunderstood. If your asking if you can go smaller than 6.5, ie. with a boom length less than 200 cm, then the answer is yes you can go to 182 cm. If anything, the boom will be most stiff at this point. There may be some added swing weight in manouvers (compared to a smaller boom), but probably of little signifance for a novice.

Hi Eric b 125;
Unreg has it correct.
The real issues with booms come when you buy a sail that's very near to the max extension of the boom.
You almost always want an extra 5-10 cm so you can add additional outhaul to the sail
to flatten it out when you are sailing overpowered.
Also, a boom at the extreme of it's extension range tends to become less stiff and the possibility of bending or breaking the boom is greater as you near the max extension.
So, yes you can get a sail that's maybe 178 cm boom length and still have enough boom (with your 182 cm boom fully collapsed) to work with.
As suggested, the boom will be at it's stiffest in this configuration, and may be heavier and have more swing weight than a shorter boom extended about half way.
Hope this helps,